Q&A

First check the valve clearances, the valves on that engine are shim adjusted and are often over looked at a "tune up"! If valve clearance is OK then check compression to make sure it is uniform and strong, perhaps do a cylinder leakage test. Distributor cap,plugs, plug wires and rotor do go bad over time. Inspect the plugs when you are doing a compression test.
With the engine running spray water from a spray bottle like Windoline/Windex on the distributor cap and plug wires if the insulation is good the idle speed should not change, if the insulation of the plug wires is bad you may hear sparking snapping sounds and hear the engine idle change .